2.23 Power Tips: Measuring Vout Ripple in DC/DC Converters

[MUSIC PLAYING] Hello, and welcome again to our low power DC/DC mini lab video series session. Today, we are going to discuss the output voltage ripple measurement of DC/DC step down converter. The output voltage ripple depends also on the external component selection, like inductor, like output capacitor, as well as on the mode of operation the device is in. Because of that, the output voltage ripple is usually not specified in the device datasheet.
However, if you need to power sensitive circuits like an IO circuit or ADC that is sensitive to the analog voltage supply, you need to know the output voltage ripple of your DC/DC converter. Therefore, you need to measure it on the evaluation board or in the application yourself.
Today we are going to do the measurement based on the TPS62135, which is the 17 volt from the step down converter, switching at 2.4 megahertz switching frequency. The measurement we are going to do is a standards CO probe, where we remove the ground clip and the ground spring minimizing any electrical fields being coupled into this CO probe.
So let's go and have a look at the output voltage ripple. Now I have modified the evaluation port and soldered two pins directly across the output capacitor. So now we can measure the output voltage directly across the output.
And now on the oscilloscope, we see the output voltage ripple. And on top of it, we have the high frequency noise caused by the switching events of the DC/DC converter. To have a good measurement, we want to remove the high switching frequency noise. And we change the scope setup. So I have already AC coupling. I move from full bandwidth to 20 megahertz bandwidth limitation for the channel. Now we can measure nicely the output voltage ripple.
In this case, the output voltage ripple, I have already set up the measurement here. It's around 18 millivolt. And this is quite good for DC/DC converter. For us, the question is now, is this setup using 20 megahertz bandwidth limitation a valid thing to do? And we could prove this by doing a measurement with full bandwidth, but we use a high frequency capacitor to decouple the high frequency noise.
And for that, I already set up the PCB. So what I have to the port now is I've soldered the coax connector directly to the output pins and use a 220 picofarad high frequency bypass capacitor directly across these pins. So this one should filter out now as well all the high frequency noise that we can measure at full bandwidth.
So I connect this CO probe. Again, across the output capacitor, there'll be a for reference measurement. And let's see whether we got rid of all the output voltage switching noise as well. Top trace-- upward voltage across the output capacitor, 20 megahertz, bandwidth limitation. Here we have full bandwidth, and the high frequency bypass capacitor removed all the noise. So that measurement just proves, then, that you can measure easily with 20 megahertz bandwidth limitation and you get very, very close to the real application results you are going to see in your application.
But now before we finish, I would like to show you a really, really cool feature on the 62135. So when I reduce the load count, the device will change the mode of operation, and all of a sudden, we'll move into pulse frequency modulation, maintaining high efficiency. But the output voltage ripple will increase as well.
So if this would violate, for instance, your application requirements because you really need low ripple, that device has a very nice feature. And I am going to show you that feature by changing the jumper position and [INAUDIBLE]. Now I force the device, and fixed frequency pulse, this modulation again. And we have a real, real, real low output voltage ripple across the entire low current range of the device. I think that is a pretty cool feature. We're
At the end again. Thanks for watching. Thanks for your time. For more information on the 62135, please go to our website, ti.com. And for more videos, please look at training.ti.com/power-tips. Thanks and see you again.

Details

Date:
July 13, 2017

The video explains the measurement set-up for the output voltage ripple in DC/DC converters. It compares the oscilloscope measurement results using 20MHz bandwidth limitation versus a full bandwidth measurement with additional high frequency bypass capacitor. It is important to verify the voltage ripple directly at the load where usually a high frequency bypass capacitor is used. The video also demonstrates the forced PWM feature of the TPS62135 minimizing the output voltage ripple over the entire load current range.